Is Lisp still worth learning today in the particular context of Machine learning?
LISP was popular because back in the old days of AI because of the functional syntax, which worked well with the GOFAI paradigm of the time. 

Nowadays most researchers have given up on the classical computational theory of mind (read: language of thought), and thus also the GOFAI paradigm that it associates with.

LISP is not what you want to learn if you want to do neural network stuff, but the philosophical background is still important to know.